Local Reflection Models CS5502 Fall 2006 (c) Chun-Fa Chang Local - - PDF document

CS5502 Fall 2006 (c) Chun-Fa Chang

Local Reflection Models

CS5502 Fall 2006 (c) Chun-Fa Chang

Local vs. Global

CS5502 Fall 2006 (c) Chun-Fa Chang

Phong Reflection Model

• I = Ka*Ia + kd*Id + Ks*Is
• Not completely correct, but good

enough. ambient diffuse specular

CS5502 Fall 2006 (c) Chun-Fa Chang

Ambient Component

• Accounting for light scatter around.
• Ia is constant.

CS5502 Fall 2006 (c) Chun-Fa Chang

Diffuse Component

• Id = Ii * N·L
• Not affected by viewing direction.

– i.e., incoming light is reflected to all directions. N L

CS5502 Fall 2006 (c) Chun-Fa Chang

Glossy (Specular) Component (Phong Reflection Model)

• To model imperfect reflection.
• Is = Ii(N · H)n

N L V H

CS5502 Fall 2006 (c) Chun-Fa Chang

Summary of Phong Reflection Model

• I = Ka*Ia + kd*Id + Ks*Is

= Ka*Ia + {Kd*(N · L)+ Ks* (N · H)n} * Ii

• Where is color? Set Ka and Kd for RGB.

Ir = Ka_r*Ia + {Kd_r*(N · L)+ Ks_r* (N · H)n} * Ii Ig =Ka_g*Ia + {Kd_g*(N · L)+ Ks_g* (N · H)n} * Ii Ib =Ka_b*Ia + {Kd_b*(N · L)+ Ks_b* (N · H)n} * Ii

• Ka and Kd depend on material color, Ks

depends on the light (which is white in the above case).

CS5502 Fall 2006 (c) Chun-Fa Chang

Phong Reflection Model (continued)

• We had:

Ir = Kar*Ia + {Kdr*(N · L)+ Ksr* (N · H)n} * Ii Ig =Kag*Ia + {Kdg*(N · L)+ Ksg* (N · H)n} * Ii Ib =Kab*Ia + {Kdb*(N · L)+ Ksb* (N · H)n} * Ii

• Alternatively:

I ={Ka*Ia + Kd*Ii*(N · L)} * object_color + Ks* Ii*(N · H)n * light_color

CS5502 Fall 2006 (c) Chun-Fa Chang

“But, they all look like plastic…”

CS5502 Fall 2006 (c) Chun-Fa Chang

Specular Component (Cook & Torrance Model)

• Consider specular reflection

as perfect reflection of micro-facets. (See Watt’s Section 7.6)

• Specular=DGF/(N·V)

D: Distribution term G: Geometry (shadowing and masking) term F: Fresnel term

CS5502 Fall 2006 (c) Chun-Fa Chang

D Term (Cook & Torrance)

• Modeling the distribution of micro-

geometry.

• Gaussian distribution can be used:

D = k e-(α/m)2

CS5502 Fall 2006 (c) Chun-Fa Chang

G Term (Cook & Torrance)

CS5502 Fall 2006 (c) Chun-Fa Chang

The Fresnel Term

• Color and ratio of

reflected/transmitted light vary with the incident and viewing angles.

• Detailed in Pharr’s

9.2.1 and Watt’s 7.6.4

CS5502 Fall 2006 (c) Chun-Fa Chang

From Watt’s color plate Figure 7.8. These would be difficult to obtain by fine- tuning the parameters in Phong model.

CS5502 Fall 2006 (c) Chun-Fa Chang

“Now, are all materials covered?” No! Let’s try a sample-based method instead…

CS5502 Fall 2006 (c) Chun-Fa Chang

BRDF

• BRDF=f(θin, φin, θref, φref)=f(L,V)

CS5502 Fall 2006 (c) Chun-Fa Chang

Watch Out for Subtly in BRDF!

• Ask yourself these questions:

– Why not just consider N · H as in the Phong’s glossy term? (Hint: Does incidence matter?) – Does φin really matter? Difference between isotropic and anisotropic reflection!

CS5502 Fall 2006 (c) Chun-Fa Chang

Why Not Always Using BRDF?

• Difficult to find a “closed form”

representation of BRDF.

• The Phong model and Cook & Torrance

model are approximation of BRDF.

– They are not 100% match of BRDF, but they are easy to compute.

CS5502 Fall 2006 (c) Chun-Fa Chang

Other Reflection Models

• Pharr’s 9.4: other microfacet models

– Oren-Nayar – Torrance-Sparrow – Blinn microfacet distribution – Anisotropic microfacet model

• Pharr’s 9.5: Lafortune model
• Models for particular materials: e.g., for

finished wood (in SIGGRAPH 2005)

CS5502 Fall 2006 (c) Chun-Fa Chang

Lafortune Model

• Phong model assumes the

glossy reflection (lobe) appears in the direction

• pposite to the incident light.
• This assumption is relaxed

in the Lafortune model.

• Multiple lobes can be used.

CS5502 Fall 2006 (c) Chun-Fa Chang

Gouraud Shading and Phong Shading

• Gouraud and Phong shadings are

interpolative techniques for rasterization.

– Polygon vertices are shaded first. – Vertex colors are then interpolated to the interior pixels.